1. FIELD OF THE INVENTION
This invention relates to a process for preparing tricyclo[ 5.3.1.0.sup.3,8 ]undecane by isomerizing 6,7-exo-trimethylenebicyclo[3.2.1.]octane. More particularly, this invention relates to a process for synthesizing easily a known tricyclic hydrocarbon, tricyclo[5.3.1.0.sup.3,8 ]undecane (II) (hereinafter sometimes referred to as "4-homoisotwistane"), by catalytically isomerizing 6,7-exo-trimethylenebicyclo[3.2.1]octane (I) as shown in the following reaction scheme (1): ##SPC1##
2. DESCRIPTION OF THE PRIOR ART
It was previously found by N. S. Vorobieva, O. A. Arefiev, V. I. Epishev and A. A. Petrov that when 6,7-exo-trimethylenebicyclo[3.2.1]octane (I) is isomerized in the presence of an aluminum halide, 1- and 2-methyladamantanes can be obtained (Neftekhimiya, 11, 163 (1971)).
We have investigated the catalytic isomerization of a variety of polycyclic hydrocarbons in the presence of acid catalysts. Isomerization of 6,7-exo-trimethylenebicyclo[3.2.1]octane (I) was examined in detail as one specific embodiment of such catalytic isomerization. As a result, it was found that in an intermediate stage of that isomerization normally leading to formation of 1- and 2-methyladamantanes as final products, a number of reaction intermediates including 4-homoisotwistane (II) are formed. These intermediates are formed at various stages of consecutive and competitive reactions starting from 6,7-exo-trimethylenebicyclo[3.2.1]octane (I) during the complicated course of these combined reactions. When the reaction is stopped partway to completion, a mixture is obtained containing variable proportions of these intermediates, depending on the reaction conditions and/or the reaction time. We found that if the reaction conditions, such as the type and amount of the catalyst, the reaction solvent, the reaction temperature and the reaction time, are appropriately chosen, the proportion of 4-homoisotwistane (II) in such intermediate mixture can be increased. This isomerization reaction can be utilized for the synthesis of 4-homoisotwistane (II), which is a known useful substance. Based on this finding, we have now completed this invention.
2. SUMMARY OF THE INVENTION
In accordance with this invention, there is provided a process for synthesizing 4-homoisotwistane (II), which comprises isomerizing 6,7-exo-trimethylenebicyclo[3.2.1]octane (I) under limited reaction conditions to form a reaction mixture containing as the main component 4-homoisotwistane (II), and separating and recovering the desired compound (II) from this mixture.
As noted above, 6,7-exo-trimethylenebicyclo[3.2.1]octane (I) used as the starting substance in this invention is a known compound which can be transformed to 1- and 2-methyladamantanes, after passing through various reaction intermediates. Accordingly, in order to obtain in a high yield the desired 4-homoisotwistane (II), which is one of these reaction intermediates, it is necessary to perform the reaction under limited reaction conditions. The term "under limited reaction conditions" means an isomerization reaction which is brought to an end at an appropriate time, an isomerization reaction which is conducted in the presence of a small amount of a catalyst or in the presence of a catalyst having a relatively low acidity, an isomerization reaction which is conducted in a solvent, an isomerization reaction which is carried out at a relatively low temperature, or an isomerization reaction which is carried out under a suitable combination of the foregoing conditions. The isomerization reaction under limited reaction conditions, according to the invention, is carried out so that the isomerization reaction is stopped at a stage at which the content of tricyclo[5.3.1.0.sup.3,8 ]undecane (II) in the reaction mixture is at least about 30 weight percent, and is preferably at the highest level that can be attained under the reaction conditions which is usually up to about 60 weight percent.
In contrast, when the isomerization reaction is carried out under severe conditions such as the use of a strong Lewis acid catalyst, for example, aluminum halides and antimony pentahalides, in an amount of more than 50 mole percent based on the starting substance (I), the absence of a solvent and a reaction temperature exceeding about 50.degree.C., the starting substance (I) is rapidly isomerized to the final 1- and 2-methyladamantanes and it is substantially impossible to detect intermediates including 4-homoisotwistane (II), the desired product of this invention.
In order to establish such limited reaction conditions, there can be used as catalyst Bronsted acids, for example, sulfuric acid, fluorosulfonic acid, chlorosulfonic acid, alkanesulfonic acids such as methanesulfonic acid, trifluoromethanesulfonic acid and ethanesulfonic acid, and arenesulfonic acids such as benzenesulfonic acid and p-toluenesulfonic acid. The amount of such Bronsted acid catalysts can be from 0.1 to 1000 moles, preferably from 1 to 100 moles, per one mole of starting substance (I). In addition there can be used as catalyst Lewis acids such as aluminum halides, boron trifluoride and antimony pentahalides, provided that the amount of said Lewis acid catalyst is from 0.01 to 0.5 moles, per one mole of the starting substance (I). As the halides, it is preferred to use the chlorides and bromides.
In order to complete the reaction in a short time, it is permissible to use a Bronsted acid in an equivalent amount or in molar excess, but in the case of a Lewis acid, in order to prevent the reaction from progressing too rapidly, the amount used of the Lewis acid should be smaller than 50 mole percent based on the starting substance.
It is possible to use a mixture comprising two or more of the foregoing acid catalysts, and in the case of some specific combinations, for example, sulfuric acid-boron trifluoride, a synergistic effect can be obtained.
In the process of this invention, advantageously good results are obtained when a solvent is used, in addition to the above selection of the amount and kind of the catalyst. The type of solvent used is not critical. Any members of the classes of aromatic hydrocarbons, aliphatic hydrocarbons, halogenated hydrocarbons and ethers can be used as solvent, provided that it is inert to the catalyst used. These solvents can be used in combination with either a Bronsted acid or a Lewis acid. The amount of the solvent is from 0.1 to 500 times the weight of the starting material (I).
In the process of this invention, the isomerization is carried out at a temperature ranging from -30.degree. to +100.degree.C., but it is preferred that the reaction be carried out at -10.degree. to +50.degree.C.
As pointed out above, N. S. Vorobieva, O. A. Arefiev, V. I. Epishev and A. A. Petrov apparently first synthesized 6,7-exo-trimethylenebicyclo[3.2.1]octane (I), the starting substance of this invention, and used it for the isomerization reaction (see the reference mentioned above), but that reference does not describe a specific process for the synthesis of this compound.
We confirmed that the compound (I) can be synthesized by any of the several processes described below. Namely, 6,7-exo-trimethylenebicyclo[3.2.1]octane (I) can be obtained, for example, by enlarging the ring of 5,6-exo-trimethylenenorbornan-2-one (III) with diazomethane and subjecting the resulting 6,7-exo-trimethylenebicyclo[3.2.1]octan-3-one (IV) to the Wolf-Kischner reduction according to the method of DeBoer et al (Org. Syntheses Coll., vol IV, 225) as shown in the following reaction scheme (2): ##SPC2##
Alternatively, compound (I) can be obtained by reducing 6,7-exo-trimethylenebicyclo[3.2.1]octan-3-one (IV) derived from a reaction product formed by ring-enlarged 5,6-exo-trimethylene-norbornene-2 (V) with dichlorocarbene according to the process of Jefford et al (Org. Syntheses, 51, 60 (1971)) as shown in the following reaction scheme (3): ##SPC3##
4-Homoisotwistane (II) prepared by the process of this invention is a compound having a tricyclic basket-like molecular structure, analogous to that of adamantane. In addition to the known utility of 4-homotwistane for preparing 1-methyladamantane, it and its derivatives are useful for pharmaceutical purposes, especially as anti-viral agents, and also are useful as lubricity-improving additives to lubricating oils and fiber-treating oils, as well as surface active agents and the like because of the inherent physical and chemical properties of polycyclic hydrocarbons.